There are many known processes for the recovery of such aldehydes from their isomeric mixtures. For example, 2-methylbutanal can be obtained by the reaction of n-butyraldehyde with formaldehyde. The reaction is carried out in the presence of alkaline reagents, under which conditions the reactants form the 2-methylenebutanal which is partially hydrogenated to the corresponding 2-methyl compound thereafter. Although this process is capable of producing a relatively pure end product, it is extremely complicated and hence expensive to carry out. Not only is the reaction in two stages, but also the partial hydrogenation requires the use of expensive noble metal catalysts.
Another method for the production of this product is by way of hydroformylation of butene-2. Naphtha cracking produces a butene-rich fraction containing compounds having 4 carbon atoms. It is known to be a by product of the manufacture of both ethylene and gasoline, and is composed primarily of butadiene, butane, butene, and isomers thereof. In particular, n-butane, i-butane, butene-1, butene-2, and i-butene are all present to a substantial degree.
The foregoing fraction, after the butadiene and the i-butene have been largely removed, is hydroformylated to produce a mixture including aldehydes having 5 carbon atoms; e.g. n-pentanal, 2-methylbutanal, and minor amounts of 3-methylbutanal.
At present, the primary means for separating the foregoing aldehydes from one another is by fractionation. Most aldehydes exhibit great sensitivity to oxidation, condensation to form higher molecular weight products, and thermal decomposition. Therefore, it is necessary that the fractional distillation process be carried out carefully. The aldehydes must be distilled off under the mildest possible conditions and only after complete removal of the catalysts. In many cases, it is necessary (or at least desirable) to use azeotropic and/or extractive distillation to minimize or avoid the unwanted reactions which lead to decomposition and condensation.
The boiling points of the components of the fraction containing the C.sub.4 compounds are extremely close together. Therefore, in order to separate them by distillation, it is necessary to have columns with extremely high separating efficiency and operate them at very high reflux ratios. These factors tend to substantially increase the cost of carrying out the separation and render such processes less economic. Moreover, there often are reductions in yield of the desired product as a result of side reactions whereby heat-sensitive, higher-boiling compounds are produced from the n-aldehydes.
In another known process, the hydroformylation of terminal olefins is carried out to produce the usual reaction mixture. The catalysts and mixture may then be separated. DE No. 24 59 152 C2 teaches that the mixture is reacted in the presence of up to a stoichiometric amount of an alkali metal hydrogen sulfite and a solvent. The sulfite precipitates the n-aldehyde as a hydrogen sulfite addition compound. This permits the precipitate to be separated, washed, and split in the usual manner.
However, because substantial manipulation of the product, as well as a considerable amount of chemicals, is required, this process is suitable primarily for the recovery of those aldehydes which are sufficiently valuable so as to justify the cost.
A thermal separation is described in DE No. 28 33 538 C2 wherein .alpha. methyl aldehydes are separated from straight-chain isomers. The aldehydes are of the formula R--CH(CH.sub.3)--CHO, wherein R is a straight-chain alkyl group having 7 to 12 carbon atoms. The thermal treatment take place in the distillation column, so that both actions take place at the same time. The branched aldehydes distill over, while the straight-chain aldehydes remain with the high boiling compounds as the residue.
Another bisulfite-based process is shown in DE No. 960 187 C1. There, the isomeric aldehyde mixture is treated with an aqueous solution of a neutral sulfite and an approximately equivalent amount of a weakly acid compound. As the mixture is heated to successively higher temperatures, the aldehydes are released in the ascending order of their boiling points and can be separately withdrawn. The remaining solution, after cooling, can be reused. However, this process is useful only for the separation of the relatively-volatile lower aldehydes; the higher aldehydes tend to decompose at or about their respective boiling points, thus rendering distillation impossible or impractical.
Yet another approach is to react the mixtures with non-oxidizing strong mineral acids. Under such circumstances, the process (as shown in DE No. 22 18 305) converts the straight-chain aldehydes into 1,3,5-trioxanes. These can then be separated by fractional crystallization. Thereafter, the trioxanes are placed in a distillation apparatus in the presence of small amounts of phosphoric oxide, thereby depolymerizing them.
As can readily be appreciated, this method requires different crystallizabilities and solubilities of the trimers of the n-alkanals and i-alkanals. When both are crystalline, the process is inoperable; as a result, the usefulness of this process is severely limited.